1. Field of the Invention
This invention relates to a motor vehicle comprising two driven axles, one of which is adapted to be driven by a drive train, which comprises a liquid friction coupling, which is preceded in said drive train by a clutch, which is adapted in a first position to cause the rear axle to be driven via said liquid friction coupling, in a second position to by-pass said liquid friction coupling and in a third position to disconnect said rear axle from said drive train.
2. Description of the Prior Art
Such a motor vehicle is already known from EP-A-O 149 302 and comprises an automatic transmission which is optionally also manually operable and does not have a special cross-country or crawling gear. The front axle is constantly driven. The coupling unit which is contained in the drive train leading to the rear axle and consisting of the liquid friction coupling and the preceding clutch must manually be operated and there is no automatic device for controlling that coupling unit in dependence on the conditions of travel. Whereas it has been stated that it is desirable to provide for a direct drive of the rear axle, i.e., for a by-passing of the liquid friction coupling, during a travel on poor roads or during cross-country travel, this is left to the skill of the driver so that difficulties may arise during travel. When the rear axle is driven via the liquid friction coupling during a travel in loose sand or deep snow, a high slip will occur at the constantly driven front axle so that the front wheels will tend to dig into the ground and will not exhibit an optimum traction behavior. Besides, the lateral force acting on the spinning wheels will greatly be reduced. Moreover, there will be a large speed difference between the halves of the liquid friction coupling under such conditions of travel so that said coupling will be subjected to high stresses. Said circumstances will obviously be particulary significant in a motor vehicle having a transmission which comprises a leat one cross-country or crawling gear because the motor vehicle is intended for prolonged cross-country travel.